The present invention is generally related to locating and/or characterizing the source of an inground transmission frequency and, more particularly, to an apparatus and method for measurement of noise that may interfere with reception of signals received at the inground transmission frequency.
In certain operations in which a transmitter is moved through the ground, substantially continuous location and orientation monitoring of the transmitter is necessary. One example of such an operation resides in the use of the transmitter being carried by an underground boring tool. Another example of such an operation resides in moving the transmitter through a pre-existing borehole or path within the ground. Operations that may use such a pre-existing path include, by way of example, the pullback of a utility line through a previously formed bore hole and mapping of various types of utility lines including water supply and waste lines. Conventional locating and monitoring systems used in conjunction with the foregoing operations are often based on well-established technology involving the detection of an oscillating magnetic field emitted by the transmitter that is moved through the ground.
One concern with respect to prior art systems relates to local interference with the transmitter signal caused by electromagnetic noise that is present in the environment. The transmitter signal is often limited to a low frequency range of less than 50 kilohertz in order for the signal to effectively penetrate the ground and be detectable by a receiver located above the surface. Several sources of noise may be present in the normal operating conditions of systems that employ a transmitter that is moved in the ground while transmitting at these frequencies. For example, underground traffic loop systems, which automatically operate stoplights according to the presence of automobiles at street intersections, can emit signals in the same low frequency range as that used for conventional locator/monitor signals. Another significant source of noise is found in the form of overhead or buried power transmission lines generally emanating noise at 50 Hz or 60 Hz (and harmonics thereof). Also, if two or more underground transmitters are operating near one another, the emitted transmitter signals may mutually interfere, thus reducing the accuracy and efficacy of all of the systems involved. Such noise sources, of which the interfering signal frequencies are known, can be referred to as urban specific noise sources. Other sources of low frequency noise may exist in the environment, such as those generated by computer network connections and community access television (CATV) lines, and these can be referred to as urban general noise sources.
Urban specific noise and urban general noise sources can limit the accuracy and the range over which an underground transmitter may be employed. For instance, the use of the underground transmitter can be restricted under streets with traffic loops. It is recognized by Applicants, however, that the limitations on accuracy and range can be frequency dependent. That is, accuracy and range at one frequency can be more limited than what is seen at a different frequency in a particular noise environment.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.